Differential display of mRNA has been used to identify genes whose expression may drive or prevent progression to tumor cell phenotype. One such gene, tissue inhibitor of metalloproteinases TIMP-3), was discovered in 1994 (Sun et al., Cancer Res, 1994; Sun et al., J Biol Chem, 1995) and found to act as a suppressor of tumor phenotype(Bian et al., Carcinogenesis, 1996). The transcriptional silencing of TIMP-3 is attributable to sequence specific methylation of its promoter (Pennie et al., Cell Growth and Diff, 1999)that when reversed restored expression of TIMP-3 to tumor cells. A separate differential display analysis was carried out to identify gene expression that may mediate or inhibit tumor promoter dependent stages of progression. This analysis identified pdcd4 (Cmarik et al., PNAS 1999) as a novel suppressor of transformation. Antisense expression of the novel pdcd4 gene converts transformation resistant (P-) to sensitive (P+) cells and pdcd4 sense expression (Yang et al Oncogene 2001) converts P+ to P- cells, thus establishing a causal relationship to prevention of tumor promoter induced transformation. Furthermore pdcd4 expression suppresses tumor phenotype in transformed JB6 cells (Yang et al Oncogene 2003). Current studies focus on the molecular function and localization of Pdcd4 protein. Examination of the possible inhibitory effect of pdcd4 on molecular events known to be required for tumor promotion revealed that pdcd4 expression inhibited the activation of transcription factor AP-1 but not of NFkappa B or of ornithine decarboxylase(Yang et al Oncogene 2001). The AP-1 inhibiting activity of Pdcd4 appears to be attributable to blocking the transactivation of cJun and cFos (Yang et al Oncogene 2003). Although expression of Pdcd4 protein blocks AP-1 activation, Pdcd4 does not interact directly with Jun or Fos proteins. Analysis of Pdcd4 binding partners by a yeast two-hybrid assay revealed the translation initiation factor eIF4A as a major binding partner (Yang et al Molec Cell Biol 2003). Binding of Pdcd4 to eIF4A is required for Pdcd4 to inhibit eIF4a's RNA helicase, to inhibit translation, and to inhibit the activation of AP-1 dependent transcription required for neoplastic transformation. Mutational analysis defines two helical MA-3 domains as required for binding to eIF4A and for inhibiting translation (Yang et al Molec Cell Biol 2004). Residues on eIF4A required for binding Pdcd4 have also been characterized (Zakowicz RNA 2005). X-ray crystallography to characterize structure in underway. Current research is focused on identifying specific transformation relevant mRNAs whose translation may be inhibited by Pdcd4 expression. Finally, we have discovered that Pdcd4 expression in human cancer cell lines is predictive for sensitivity to tamoxifen and geldanamycin. Moreover, expression of Pdcd4 actually confers sensitivity to these drugs (Jansen et al Molec Cancer Ther 2004).